1. Field of the Invention
This invention relates to coatings for implantable electrodes such as pacing electrodes, neurostimulator electrodes, electroporating electrodes, and sensing electrodes. More particularly, the present invention is directed to the creation of additional porosity and thereby additional surface area in an implantable electrode.
The three overriding requirements for implantable electrodes are biocompatibility, biostability, and low energy loss during tissue stimulation. Broadly, the biocompatibility requirement is met if contact of the electrode with body tissue and blood results in little or no immune response from the body, especially thrombogenicity (clotting), infection, and encapsulation of the electrode with fibrotic tissue. The biostability requirement means that all physical, electrical, and chemical properties of the electrode/coating system remain constant and unchanged over the life of the patient. The low energy loss requirement is met if electrode polarization is optimized.
2. Prior Art
U.S. Pat. No. 4,602,637 to Elmqvist et al. teaches that upon stimulation of body tissue the polarization rise of an active surface layer is maintained “very slight” by use of a high double layer capacitance at the phase boundary between the electrode/body fluid. The high double layer capacitance maintains the polarization rise during stimulation pulses (0.5 through 1 ms, 1 Hz, 10 mA, 10 mm2) to less than 0.1 V. This is accomplished through high specific surface area coatings, such as of titanium nitride (TiN), by specifying processing parameters. These parameters utilize low adatom surface mobility and an increase in liberated species collisions to produce pronounced columnar structures of the TiN via physical shadowing. Due to the low surface mobility, discrete nucleation sites are formed on the substrate. During subsequent vapor flux arrival, the already existing deposits physically shadow the un-reacted substrate. Physical shadowing by the major constituent leads to columns.
Other prior art processes for producing electrode coatings exhibiting low polarization rises during stimulation pulses are accomplished by increasing the specific surface area at the phase boundary between the electrode/body fluid. This involves removal of material from an already coated electrode surface. The coating (or bare substrate if no coating is used) is subjected to etching via electrochemical or chemical, ionic or physical means. In the case of electrochemical and chemical etching, an agent that leaves holes by preferentially attacking the major constituent is used. The difference between electrochemical and chemical etching is that the former includes an electrolytic bath in which an electrical bias helps with the etching process and the latter does not.
In the case of ionic cleaning or etching, the surface of the coating is bombarded with ions, thus preferentially etching the areas of low radius. This ionic etching can also incorporate a screen for imparting patterns on the surface. In the case of physical etching, mechanical means are used to remove surface layers and increase surface area. This is done by employing techniques such as laser machining and grit blasting.
For a better understanding of electrodes having features imparting high specific surface area, reference is made to the drawings. Throughout this disclosure, the term “specific surface” refers to the ratio between all surface areas that are capable of undergoing electrochemical activity while in service and the geometric surface area of the exposed part of the electrode body. This includes surface roughness, porosity, and convolution.
The porosity of a coating consists of three types of porosity features, macroscopic, microscopic and nanoscopic. A macroscopic surface has details characterized by features ranging from about 10 μm to about 1,000 μm. Microscopic features range from about 100 nm to about 1000 nm while nanostructures have features of less than about 50 nm.
FIGS. 1 and 2 show a conventional electrode 10 comprising a shaft 12 joined to a substrate in the form of a head 14 as a unitary member. The electrode 10 is of a material selected from tantalum, titanium, zirconium, iridium, platinum, palladium, niobium, and mixtures thereof. Preferably, the electrode is of platinum/10% iridium. Although the shaft 12 is cylindrical, that is not necessary. Also, the electrode head 14 is shown as a dome-shaped member of a constantly curved radius, but that is also not necessary. That the electrode has a head 14 providing a surface, coated or otherwise, that is capable of a low energy loss transmission of electrical energy into a body tissue is what is relevant.
FIGS. 3 and 4 illustrate a similar conventional electrode 20, but one that has been subjected to mechanical treatment. This electrode 20 comprises a shaft 22 joined to a substrate in the form of a dome-shaped head 24 as a unitary member. However, the head 24 has been provided with a grooved cut-out 26 that is generally centered along the longitudinal axis of the shaft 22. That the groove 26 is centered with the shaft or that there is only one groove is not limiting. The point is that mechanical means such as machine cutting, laser cutting, etching, grit blasting, and the like have been used to increase the active surface area of the head 24 in comparison to the electrode 10 shown in FIGS. 1 and 2.
FIGS. 5 and 6 illustrate an electrode 30 comprising a shaft 32 extending to a head 34 as a unitary member. In that respect, it is similar to the electrode 10 of FIGS. 1 and 2. However, the head 34 has been provided with macroscopic surface structures 36 by the addition of particles ranging in size from about 10 μm to about 1,000 μm. The macroscopic surface structures 36 can be any material that has high biocompatibility, biostability, and electrical conductivity. Examples include carbon, boron, platinum, palladium, iridium, gold, titanium, tantalum, niobium, ruthenium, zirconium, and alloys thereof. In addition, the carbides, nitrides, carbonitrides, and oxides or doped oxides of these metals, and their alloys, may be used including iridium oxide, iridium nitride, titanium nitride, titanium carbide, titanium carbonitride, tantalum nitride, tantalum carbide, tantalum carbonitride, niobium carbide, niobium nitride, niobium carbonitride, ruthenium oxide, ruthenium nitride, zirconium oxide, zirconium nitride, zirconium carbide, and mixtures thereof. In cases where the compounds of the macroscopic surface materials 36 are not electrically conductive, they can be made so by doping with small amounts of extraneous elements. For example, titanium dioxide, a dielectric in its pure state, is made conductive by doping with niobium. Titanium nitride is a particularly preferred material for the macroscopic surface structures 36.
All of these macroscopic surface materials 36 can be applied to the electrode head 34 in such a way that the resulting coatings have high surface areas with very fine scale roughness and porosity. Suitable deposition methods include physical vapor deposition processes such as sputtering (deposition by plasma activation), evaporation (deposition by thermally activated vaporization), pyrolytic deposition (thin film thermally deposited by decomposing a liquid precursor), or by chemical vapor deposition (thin film thermally deposited by decomposing a gaseous precursor).
The electrode 20 of FIGS. 3 and 4 exhibits improved polarization upon stimulation in comparison to the electrode 10 of FIGS. 1 and 2, primarily due to its mechanical structures, i.e. the groove 26. Further, the electrode 30 of FIGS. 5 and 6 exhibits improved polarization in comparison to electrode 20. This is due to the macroscopic surface materials 36 supported on the head 34.
It has been shown that by increasing the specific surface area of a coating, for example a coating of titanium nitride (TiN), the polarization of an electrode can be reduced. Referring again to the drawings, FIG. 7 shows a conventional electrode 40, similar to electrode 30 of FIGS. 5 and 6, comprising a shaft 42 extending to a head 44 provided with microscopic surface structures 46 in the form of columns 48. Exemplary materials for the microscopic surface structures 46 are the same as those of the macroscopic materials 36, with columnar titanium nitride being preferred.
There are two types of porosity that lead to the formation of specific surface area, namely inter-columnar and intra-columnar. As shown in FIG. 8, inter-columnar porosity 50 is formed by voids left between columns 48. This microscopic porosity is seen upon low-resolution investigation, i.e. a magnification of about 2000×. As shown in FIG. 9, intra-columnar porosity 52 is formed within each column 48 by dendrite structures 54 (FIG. 10). This nanoscopic porosity appears as a feathery structure under high-resolution investigation, i.e. a magnification of about 30,000×. It is therefore conceivable that the total porosity of a conventional coating system may be predominately governed by intra-columnar nanoscopic porosity 52 formed in the dendrite structures 54. Small increases in this porosity may lead to ten fold increases in specific surface area. The coated electrode illustrated in FIGS. 7 to 10 yields a total surface area of about 1 mm2 to about 20 mm2.
However, there is still a need for an implantable electrode having the requisite biocompatibility and biostability characteristics, such as provided by columnar titanium nitride, but that advances the state of the art through high specific surface characteristics. The result is an electrode with a lower polarization rise upon stimulation than is currently provided by columnar titanium nitride, and the like. The present electrode fulfills this need in terms of both low polarization and minimum energy requirements for acceptable sensing properties by the incorporation of secondary nanoscopic structures supported on the columnar microscopic structures 48.
The foregoing and additional objects, advantages, and characterizing features of the present invention will become increasingly more apparent upon a reading of the following detailed description together with the included drawings.